This invention relates to a method of combustion for depressing the nitrogen oxide content of the exhaust gas discharging from a combustion chamber. More particularly the present invention relates to a method of combustion in a combustion chamber, which method comprises subjecting the nitrogen oxides (hereinafter referred to NO.sub.x) occurring during the combustion of fuel to an additional combustion using hydrogen and/or carbon monoxide for thereby causing the nitrogen oxides to be reduced and brought to extinction and consequently depressing the escape of nitrogen oxides in the exhaust gas.
Of the techniques for controlling NO.sub.x discharge from the combustion chamber, the most widely known are the flue gas partial recirculation method and the two-stage combustion method. Because the occurrence of NO.sub.x is most intensified in the high-temperature oxidizing flame, these techniques are invariably directed to simultaneously lowering the concentration of oxygen which goes to form the flame and holding down the temperature of combustion. Since the high-temperature oxidizing flame enjoys the highest combustion efficiency, efforts to preclude the state of high-temperature oxidizing flame entail numerous disadvantages.
In the case of the flue gas recirculation method, for example, the stable state of combustion is sacrificed when the recycling ratio of the flue gas is increased for the purpose of providing enhanced depression of the NO.sub.x formation. In the case of the two-stage combustion method, the fuel fails to undergo complete combustion and gives off black smoke when the amount of air supplied relative to the fuel in the initial stage of combustion is decreased for the purpose of more rigidly controlling the occurrence of NO.sub.x. Particularly when there is used coal or heavy oil which is quite productive of NO.sub.x, the extent to which the amount of air supplied in the initial stage of combustion can be decreased is rather limited because of inferior combustion. Thus, this method cannot be expected to provide satisfactory depression of the NO.sub.x occurrence.
It is, therefore, an object of this invention to provide a method of combustion which provides effective depression of the occurrence of NO.sub.x while permitting the combustion to proceed efficiently in the form of high-temperature oxidizing flame.